Marine Subsidies Produce Cactus Forests on Desert Islands

Marine nutrient subsidies can shape terrestrial plant biodiversity. In island systems, nitrogen-rich seabird guano is a large component of such marine subsidies. In zones of nutrient upwelling such as the Gulf of California, copious seabird guano is commonplace on bird islands. Several bird islands host regionally unique cactus forests, especially of the large columnar cactus, cardón (Pachycereus pringlei). We propose that a chain of interactions across the land-sea interface yields an allochthonous input of nitrogen in the form of seabird guano, fueling the production of some of the densest cactus populations in the world. Fish, seabird, guano, soil, and cactus samples were taken from Isla San Pedro Mártir for nitrogen stable isotope ratio measurements, which were compared to soil and cactus samples from other seabird and non-seabird Gulf islands and terrestrial ecosystems throughout the range of the cardón. Isla San Pedro Mártir δ15N values of the food/nutrient cycle are distinctively high, ranging from fish +17.7, seabird +19.7, guano +14.8, soil +34.3 and cactus +30.3. These δ15N values are among the highest ever reported for plants. Seabird island soil and cactus δ15N values were consistently enriched relative to mainland and non-bird islands. Our findings demonstrate that seabird mediated marine nutrient deposits provide the source for solubilized N on desert islands, which stimulate terrestrial plant production in the cardón cactus significantly beyond that seen in either mainland ecosystems or non-seabird islands. These results elucidate the integral nature of nutrient movement across the land-sea interface.

Seabird Transfer of Nutrients and Trace Elements from the North Water Polynya to Land during the Mid-Holocene Warm Period, Carey Islands, Northwest Greenland + Supplementary Appendix Figure S1 (See Article Tools)

Seabird guano from large nesting colonies is known to increase trace metal levels in adjacent terrestrial environments today, when global oceans are contaminated with Hg, Cd, and other metals. But the effect of seabird guano in the pre-industrial period has rarely been studied. We used stable C and N isotopic and trace element analyses of a peat core that represents ca. 2000 years of organic matter accumulation to examine the effect on trace elements and nutrients of a seabird colony that existed in northern Baffin Bay during the Holocene Thermal Maximum (ca. 8000–5000 yr BP). Although C and N concentrations were typical of those in other peats, isotopic data identified marine organic matter as the main source of N and a minor source of C in the peat and showed that the unknown seabird was a fish-eating species that was summer-resident for the 2000 yr period. Concentrations of Cd, Br, Sr, and Zn in peat were up to an order of magnitude higher than in ombrotrophic (air-fed) bogs elsewhere, whereas Hg and Cu concentrations were similar to those in other peats, suggesting relatively low levels of Hg and Cu in the guano. This surprising result for Hg contrasts with studies on modern seabirds, in which guano markedly increased environmental Hg concentrations. It could be a consequence of Hg concentrations in Arctic marine food webs in the pre-industrial period that were an order of magnitude lower than those of today.